As optical systems continue to increase the volume and speed of information communicated, polarization controllers are becoming increasingly important optical networking elements. For example, polarization controllers are essential in polarization multiplexed lightwave transmission systems. These systems can operate in a number of ways. In one embodiment, alternate bits can be polarized orthogonal to one another and combined to create a faster overall transmission rate. In another embodiment, densely packed adjacent wavelengths can be orthogonally polarized to minimize interaction between the adjacent wavelengths. In either case, a polarization controller is used to appropriately align the signals, states of polarization.
As another example, polarization controllers can be useful in upgrading the operation of polarization sensitive optical components. Where an optical component's performance changes depending on the state of polarization of the signal it processes, a polarization controller can be used to align the signal's state of polarization with the state that maximizes the device's performance.
Polarization controllers also find application in devices used to mitigate polarization mode dispersion arising in optical signals. Most all optical fibers exhibit non-circular—typically elliptical—core shapes, which result in the fiber having two principal axes having different modal indices. The orientation of these axes varies randomly with position and time. Signals polarized parallel to the two principal axes experience differential delay, which—coupled with the random variation in polarization modes—leads to pulse broadening, intersymbol interference, and bit error ratio (BER) impairment. These types of phenomena are typically referred to as polarization mode dispersion. Polarization mode dispersion can limit an optical system's transmission range by 1/R2, where R represents the system's channel rate. Many communication systems consider unacceptable any pulse broadening greater than ten percent of the bit period. As a result, it has been estimated that polarization mode dispersion renders over twenty percent of all currently deployed fiber unsuitable for transmission at ten Giga-bits per second, and over 75% of all installed fiber unsuitable for transmission at forty Giga-bits per second. Polarization controllers can be used in polarization mode dispersion compensators, for example, to help align the principal states of polarization with appropriate axes of a polarization delay line.
Various techniques have been devised to attempt to control or modify the state of polarization of optical signals. For example, butterfly polarization controllers exist consisting of multiple rings of fiber that are physically rotated with respect to each other. This approach, however, is too slow to be effective for most applications. Another approach is to mechanically squeeze the fiber at strategic locations and times. This technique is also typically to slow to be of practical use. Lithium niobate based polarization controllers have been produced that exhibit acceptable speeds. However, these devices can be prohibitively expensive, even in a single wavelength application.
Another approach uses polarization rotators constructed from micro-machined movable mirrors to help rotate the state of polarization of an incoming signal. This approach suffers, however, because it requires either physical rotation of the polarization rotators, or requires insertion of bulk wave plates between each of the polarization rotators. These limitations make it difficult, if not impossible, to package arrays of the polarization controllers, and can result in high fabrication costs. The design and fabrication cost of these devices generally renders them unsuitable for multiple wavelength applications.
Another device that is somewhat related to a polarization controller, which is designed for integrated waveguide implementation, uses two phase shift stages coupled to a variable delay line. This approach suffers because requiring a variable delay line typically results in greater expense than a fixed delay element, and generally requires more complex and expensive control circuitry.